Well the two fostered chicks came out of the nest the other day and shortly thereafter the green chick died. :(
There seem to be two "danger zones" times for Gouldian chicks.
The first is - get hatched but don't get tossed. If the chick isn't tossed on the very first day it hatches then usually it will be okay.
The second is - get out the nest successfully. Gouldian chicks seem to be especially vulnerable at this time. They need to keep themselves warm, get fed and learn fly around without injuring themselves.
From my observations over time, these two foster chicks have exhibited subtle differences from the chicks that I have seen that were fully parent raised. The major thing I've noticed is that these chicks don't seem to be as good at begging for food. Godric and Minerva's own chicks are very loud and very persistent. In fact, after leaving the nest, the chicks would go so far as to almost harass their parents, following them everywhere and constantly begging.
The foster chicks seem much lazier and less robust. Even in the nest, their begging was less frequent and less pronounced. And now out of the nest, the blue baby usually only begs when Godric or Minerva happen to come near him. And his begging is more half hearted. If he gets ignored, the chick just seems to accept it. Godric and Minerva seem to be doing their best. I've seen both of them initiate feeding the chick so I don't think it is really their fault.
We had a power outage yesterday so I spent an hour or so observing all of the birds closely. The green chick had already died and I was very concerned about the temperature falling too low for the blue chick. I had taken away then nest as soon as the chicks had fledged (were out of the nest) because I didn't want to encourage Godric and Minerva to abandon the foster chicks to start a new clutch of their own. Fortunately, the power came back on before night fall so I was able to add a little heat lamp to help the blue chick rally.
Cross your fingers - so far he seems to be hanging in there. It could go either way at this point. I don't think he will really be out of danger until he learns to feed on his own. I just hope he will be tough enough to get what he needs until then.